Taking its first major step to create a fourth television network, Rupert Murdoch's Fox Broadcasting Co. yesterday announced that it will launch a live late-night talk show hosted by comedienne Joan Rivers this fall to compete with Johnny Carson's Tonight Show on NBC.

"We snatched Joan away from NBC," said Fox Inc. Chairman Barry Diller.

The late-night entry is the first programming salvo in what Murdoch and Diller hope will become a fourth television network consisting of independent television stations -- that is, stations not affiliated with any of the three major networks or the Public Broadcasting System.

Fox owns television stations in Washington, New York, Chicago, Los Angeles, Dallas and Houston.

The number of independent television stations has more than doubled during the last five years to more than 250, according to the Independent Television Association; such stations now reach more than 80 percent of U.S. households with television sets, compared with a 60 percent coverage level seven years ago.

Diller, who went to Fox from running Paramount Pictures, disclosed in a recent interview that the Fox Network initially would offer independents 10 hours a week of programming beginning in March 1987 -- including two nights a week of prime-time programs plus late-night shows.

In its first year, the network would reach 65 percent of U.S. households and grow in coverage by an additional 5 percent a year, according to Diller.

"We're trying to get an 8 [rating] in our universe of stations in prime time," said Diller.

Paul Bortz of Brown, Bortz and Coddington, a television consulting firm based in Denver, questioned Diller's target. "That's an extremely ambitious program, and I would question whether they could achieve that," he said. "It's not good enough to be on stations that reach 65 percent of the country -- those have to be good stations with good signals. You get into trouble just using the coverage numbers -- you have to look at the quality of outlets."

However, late-night programming can be profitable with far lower ratings than required for prime time success.

The Alan Thicke late-night show was launched in 1983 but canceled in less than a year, however, after several ratings of under 1 point.

Fox's capture of Rivers -- who has been a Tonight Show guest hostess for several years -- is in the tradition of networks stealing away talent. William Paley's CBS, then trailing in the ratings, received a huge ratings boost after it successfully pirated away NBC star Jack Benny in the 1950s.